1. Field of the Invention
The present invention relates in general to a spatial light modulator used for optical information processing, image processing or X-ray image detection. More particularly, this invention is concerned with improvements in such a spatial light modulating element which uses a single crystal plate having both electrooptic effect and photoconductivity, and which is capable of performing various functions such as, conversion of an incoherent optical image into a coherent optical image, optical information processing, image processing, optical logical operation, or conversion of an X-ray image into a visible light image.
2. Discussion of the Prior Art
Studies and researches have been made on single crystal plates, such as bismuth silicon oxide (BSO=Bi.sub.12 SiO.sub.20) and bismuth germanium oxide (BGO=Bi.sub.12 GeO.sub.20), which have both electrooptic and photoconductive properties or effects, in an attempt to utilize these single crystal plates as image converting elements, so-called PROM elements (Pockel's Readout Optical Modulator elements), for optical two-dimensional image or information processing devices. A construction of the PROM element is illustrated in FIG. 1, wherein two insulating layers 4, 4 formed of polyparaxylene, for example, are provided on opposite surfaces of a BSO single crystal wafer 2, and two light-transparent electrodes 6, 6 formed of indium oxide, for example, are provided on the respective insulating layers 4, 4.
Usually, the recording of information or images in such a PROM element is effected in the following manner. Initially, a voltage of an external power source 8 is applied between the electrodes 6, 6 on the opposite surfaces of the BSO crystal wafer 2, to produce an uniform electric field within the crystal, in a direction normal to a crystallographic plane. In this state, the crystal wafer 2 is exposed to a blue radiation (having a wavelength of about 450 nm), to form an image in the crystallographic plane. Since the BSO crystal 2 exhibits high photoconductivity upon exposure to the blue wavelength range of light, carriers are induced within the crystal, depending upon the amounts of local exposure of the crystal to the blue radiation. The carriers move through the crystal 2 and reach the insulating layers 4, by means of the electric field produced by the applied voltage. As a result, a distribution of the photoinduced charges corresponding to a specific distribution of the local exposure amounts is formed. The electric field strength in the crystal 2 is reduced due to the electric field formed by the charges, whereby a distribution of the electric field strength corresponding to the light exposure distribution is formed. Thus, information or an optical image represented by the blue radiation is recorded in the BSO crystal 2.
The reproduction or readout of the image thus recorded in the BSO crystal wafer 2 is effected by utilizing an electrooptic effect of the BSO crystal. More specifically, the BSO crystal 2 has birefringence or double refraction of an incident light due to the electrooptic effect, the degree of the birefringence being proportional to the intensity of the electric field formed within the crystal. It is noted that the two principal birefringent axes of the BSO crystal 2 are perpendicular to the direction of the electric field. In FIG. 2, there is indicated at 12 the PROM element which includes the BSO crystal 2 whose principal birefringent axes are indicated by respective two arrows. In reading out the information from the BSO crystal 2, a linearly polarized red radiation (having a wavelength of about 650 nm) is incident upon the PROM element 12 through polarizer 10, such that the plane of polarization of the red radiation forms 45.degree. with respect to the two principal birefringent axes of the BSO crystal 2. As a result, the incident linearly polarized red radiation is converted into an elliptically polarized radiation, according to the distribution of the local electric field strength in the BSO crystal 2. Since the level of the photoconductivity of the BSO crystal 2 when exposed to the red radiation is low, the image recorded in the crystal is not destroyed or influenced by the readout red radiation. As also indicated in FIG. 2, an analyzer 14 is disposed on the output side of the PROM element 12, such that the analyzer 14 and the polarizer 10 constitute a crossed arrangement. The analyzer 14 emits an optical output whose intensity corresponds to the ellipticity of the incident elliptically polarized radiation, whereby the image recorded in the PROM element 12 is read out by the red radiation.
The erasure of the image recorded in the BSO crystal 2 of the PROM element 12 is effected by irradiating the BSO crystal with a strong blue radiation, with a uniform intensity.
The light-transmitting type PROM element constructed as shown in FIG. 1 may be replaced by a PROM element of a light-reflecting type as shown in FIG. 3, wherein a reflecting electrode 16, formed of a metallic or other material capable of reflecting light, is provided on one of opposite surfaces of the BSO crystal 2, so that an incident readout radiation is reflected by the light-reflecting electrode 16. On the other of the opposite surfaces of the crystal 2, there is provided the insulating layer 4 on which is provided the light-transparent electrode 6.
The PROM element constructed as shown in FIGS. 1 and 3 exhibits photoconductivity with respect to X rays, too. Namely, the X rays may be used for writing or recording an input image in the BSO crystal of the PROM element, and the input X-ray image may be converted into a visible light image. It is also noted that the PROM element may use a BGO crystal, in place of the BSO crystal, for achieving the same function as indicated above.
The insulating layer or layers 4 is/are essential to the PROM element, as described above. The material for the insulating layer 4 must be selected so as to satisfy the following requirements. That is, the insulating material must be easily processed, permitting stable fabrication of the insulating layer with a uniform thickness. Further, the material is required to provide for a sufficiently small thickness and a sufficiently high dielectric constant of the insulating layer.
In the case where the PROM element 12 having the two insulating layers 4 on the appropriate opposite surfaces of the BSO crystal 2, the ratio of a voltage Vbs.sub.0 applied to the BSO crystal 2 to the entire voltage V.sub.0 applied to the PROM element 12 is represented by the following equation: EQU V.sub.0 Vbs.sub.0 =1+(2.epsilon.bs.sub.0 .multidot.dg/dbs.sub.0 .multidot..epsilon.g)
where,
.epsilon.bs.sub.0 =dielectric constant of the BSO crystal PA0 dbs.sub.0 =thickness of the BSO crystal PA0 .epsilon.g=dielectric constant of the insulating layer PA0 dg=thickness of the insulating layer
For efficient voltage application to the PROM element with a minimum voltage drop at the insulating layer, the element must be designed such that the ratio V.sub.0 /Vbs.sub.0 is as close to "1" as possible. It will be understood from the above equation that the ratio approaches "1" as the value dg decreases. It is therefore desirable that the thickness of the insulating layer be as small as possible for efficient voltage application. It is also known that the value dg is desirably small for improved resolution of the image to be read out.
For the same reason as explained above, the ratio V.sub.0 /Vbs.sub.0 approaches "1" as the value .epsilon.g increases, and it is consequently desirable that the dielectric constant be high. It is also known that the value .epsilon.g is desirably large for improvement of the image resolution.
Further, the material of the insulating layer desirably has the following properties: high resistance to the applied voltage (particularly where the thickness is small); high insulation resistance; high light transmitting property, without birefringence; and high chemical resistance, high weather proof or environmental resistance and high heat resistance, for high durability and excellent operating reliability.
Several materials for the insulating layer have been proposed, as disclosed in laid-open Publication Nos. 54-48262, 57-49916 and 63-110416 of unexamined Japanese Patent Applications, which teach the use of organic insulating materials such as polyparaxylene and polystyrene, and inorganic insulating materials such as MgF.sub.2, mica, and isotropic single crystals of oxides such as Bi.sub.4 Si.sub.3 O.sub.12 and Bi.sub.4 Ge.sub.3 O.sub.12.
However, the use of polyparaxylene (organic insulating material) for the insulating layer results in increased difficulty in forming a transparent electrode by sputtering on the insulating layer, and increased complexity in construction of the PROM element obtained. More particularly, since polyparaxylene has a low melting point and a low heat resistance, a DC sputtering process must be practiced for forming the transparent electrode such as an indium oxide film on the insulating layer, in order to accurately control the sputtering conditions such as cooling of the BSO crystal, Ar or O.sub.2 gas, magnetic field in the relevant space and sputtering power. Further, since polyxylene tends to be easily deteriorated under a humid condition, the insulating layer must be protected by a gas-tight enclosure filled with a dry nitrogen gas. The provision of such an enclosure makes the PROM element considerably complicated in structure.
The use of polystyrene, on the other hand, results in insufficient mechanical strength of the insulating layer, leading to relatively short life expectancy of the PROM element.
Where mica as an inorganic insulator is used for the insulating layer, the birefringent property of the mica per se causes a phase difference of a linearly polarized light used for reading the recorded information by means of an electrooptic effect of the BSO crystal. As a result, the contrast ratio of the readout image is undesirably low. Further, the mica insulating layers have a relatively low dielectric breakdown voltage because of strength and is easily structurally deteriorated due to pin holes. Moreover, since the thin insulating layer of mica is obtained by utilizing the basal cleavage of the mica, the mica insulator is difficult to control its thickness to a desired nominal value, whereby the PROM elements using the mica insulating layers tend to suffer from fluctuation in the operating characteristic and performance.
In the case of using MgF.sub.2 for the insulating layer, the insulation resistance is as low as about 10.sup.10 .OMEGA.cm, and the mechanical strength is insufficient, with the layer easily absorbing humidity. Where Bi.sub.4 Si.sub.3 O.sub.12 or Bi.sub.4 Ge.sub.3 O.sub.12 crystal is used, the dielectric constant is about 16 and is not sufficiently high.
As described above, the conventionally used materials for the insulating layer suffer from several drawbacks, which include: low insulating stability of the insulating layer; difficult control of the thickness of the insulating layers; consequent complexity of the PROM elements obtained; and low contrast ratio of the readout image.
OPTIC COMMUNICATIONS, Volume 71, number 1, 2, p.29-34, May 1, 1989, J. Chen and T. Minemoto shows a PROM element which uses as an insulating layer a uniaxial single crystal of oxide which is oriented such that the crystal exhibits an electrooptic effect. Because the electrooptic effects in both an electrooptic and photoconductive BSO crystal and the insulating layer are utilized at the same time, the PROM element has a comparatively low halfwave voltage (V.pi.). However, the PROM element obtained suffers from insufficient readout contrast of a positive image. Further, since the electrooptic effect of the insulating layer is also utilized, the readout operation is influenced by an angle of incidence of the readout light beam, and the PROM element is accordingly difficult to use.